1. Field
The present invention relates to a method for manufacturing a polyimide carbon nanofiber electrode and/or a carbon nanotube composite electrode, and a CDI apparatus using the electrode. More specifically, the present invention relates to a method for manufacturing a flat plate electrode from polyimide carbon nanofibers without using any binder and a CDI apparatus using the flat plate electrode.
2. Description of the Related Art
Generally, carbon fibers and activated carbon fibers are classified into polyacrylonitrile (PAN)-based, acryl-based, pitch-based, phenol-based carbon fibers, etc., depending on the starting material to make the fibers.
Carbon fibers are prepared using wet-, melt- or dry-spinning that employs melting PAN-based, acryl-based, pitch-based, or phenol-based polymers, etc., at ambient temperature or a high temperature and then drawing or pulling out fibers from the molten polymers at a physical pressure.
Meanwhile, activated carbon fibers are prepared by activating carbon fibers with water vapor, carbon dioxide, KOH, ZnCl2, etc.
The carbon fibers prepared by these traditional spinning methods almost have a relatively high diameter of about 5 to about 50 μm. Due to the high diameter, the carbon fibers have a low flexural strength and are not thus easy to apply to compress processing.
In recent years, electrostatic spinning (also called “electrospinning”) has been used, which is a method capable of preparing ultrafine fibers from polymers via an electrostatic force, in contrast to the spinning methods depending upon a physical force. In accordance with the electrostatic spinning, a polymeric solution, to which a high-voltage electric field is applied, is sprayed to prepare fibers. More specifically, positive (+)-charged ions in the polymeric solution are discharged from an ejector and then adsorbed on a negative (−)-charged electrode collector to produce a nanofiber web.
The preparation of carbon nanofibers or activated carbon nanofibers using the electrostatic spinning is carried out by dissolving PAN, pitch or phenol in a solvent such as metacresol, subjecting the resulting solution to electrostatic spinning to prepare carbon nanofibers, and stabilizing, carbonizing or activating the carbon nanofibers.
For example, Korean Patent Laid-open Publication No. 10-2003-0089657 discloses preparation of carbon fibers and activated carbon fibers from polyamic acid (PAA) by electrostatic spinning and its applications to electric double layer supercapacitor electrodes.
More specifically, the afore-mentioned publication paper discloses preparation of polyimide fibers having nanometer-scale diameters with superior electrical conductivity by electrostatic spinning, preparation of carbon nanofibers and activated carbon nanofibers from the polyimide fibers, and the use thereof as electric double layer capacitor electrode materials.
Polyimide is a highly thermal and chemical resistant polymer having an imide group in the repeat units, imide monomers. In spite of these advantages, polyimide has limited applications. The reason is that polyimide has poor processability into a specific shape due to solvent-insolubility and heat resistance (flame resistance).
Accordingly, polyimide processing is carried out by processing polyimide into a specific shape in a PAA precursor solution using a polar solvent and converting the polyimide into imide using a thermal or chemical method.
Thus, the invention disclosed in the publication paper suggests a method for preparing nanometer-scale ultrafine carbon nanofibers and activated carbon nanofibers with a high specific surface area by electrostatic-spinning polyimide with superior electrical conductivity, and applications thereof to an electric double layer capacitor electrode without using any binder.